In the context of offshore hydrocarbon production, a riser is a tubular conduit that extends from a subsea structure to a production platform, for the purpose of conveying produced hydrocarbons from the well to the platform. The subsea structure may be a “christmas tree”, riser base, Blowout Preventer (BOP), or some other structure. Flexible risers are particularly important for deep water wells.
Risers are configured in order to allow for movements of the platform relative to the subsea structure in both a horizontal and a vertical direction, and for the effects of ocean currents on the riser itself. Configurations include a “free hanging” configuration (where the riser extends substantially vertically down to the seabed and then horizontally along the seabed to the subsea structure, a reverse pliant wave configuration, a pliant wave configuration, a lazy S configuration, and other suitable configurations.
For riser removal and installation operations on platforms where the risers are exposed/hanging on the platform side, the normal methodology involves the use of an installation vessel (IV) in addition to the platform itself. The IV is equipped with industry-standard laying spread (normally Vertical Laying System—VLS) and reels/carousels/baskets for storage of risers. The IV can install the riser from both directions, i.e. either the subsea or the topside end of the riser may be installed first (subject for example to already in-place riser configuration, auxiliary equipment size, packing constraints etc.).
FIGS. 1 to 6 illustrate schematically various stages in the riser installation process in the case of floating production storage and offloading (FPSO), where the riser is being installed to achieve a reversed pliant wave configuration. Very generally, the process involves the following steps:    (1) An end of a pull-in wire 101, suspended from the platform (attached to the seabed by tethers 110), is transferred from the platform to the installation vessel (FIG. 1). FIG. 1 also shows an existing riser 111.    (2) The transferred end of the pull-in wire 101 is attached to the bottom of the riser 102 such that the pull-in wire 101 is now suspended between the platform and the riser 102. This bottom end of the riser 102 will subsequently be attached to the platform and so is referred to hereinafter as the “topside” end of the riser 102.    (3) The riser 102 is fed into the water from the installation vessel (FIG. 2). During this stage, the installation vessel steps away from the platform.    (4) At this stage, the riser 102 has adopted a U-shaped configuration in the water (FIG. 3). At some intermediate point in this procedure, buoyancy modules 103 are installed around the riser, over a certain length (FIG. 4). A pull-down clump weight (PDCW) may be required to pull-down the buoyant section in this phase.    (5) When the riser 102 has reached a certain depth into the water, the topside end is pulled up to the platform using the pull-in wire 101. The topside end of the riser is then installed to the appropriate connection on the platform (“hang-off”).    (6) The installation vessel continues to lower the riser 102 and, as a result of the buoyant section of the riser, the curvature inverts (FIG. 4).    (7) The installation vessel then continues to lower the riser 102 into the water, stepping further away from the platform as it does so (FIG. 5).    (8) The installation vessel installs anchor clamp(s), subsea bend restrictors, subsea tie-in head etc., onto the riser 102 and lowers the subsea end of the riser 102 into the water, suspended by a winch cable. The riser 102 is normally required to be pulled-down, by means of the PDCW or pull-down rope/wire.    (9) When the riser 102 has been lowered to a position over the (previously installed) seabed anchor(s), an intermediate point on the riser 102 is connected to the anchor(s) via tether(s), typically using a remotely operated vehicle (ROV).    (10) The installation vessel steps further away from the platform, laying the riser 102 along the seabed until, finally, the subsea end of the riser is located in close proximity to the subsea structure to which it is to be connected (FIG. 6, which shows only seabed layout of the new riser). The winch cable is then disconnected.    (11) The subsea end of the riser may be connected to the subsea structure using an appropriate tie-in tool and/or divers and/or an ROV.
WO2011/099869 describes a method of installing a riser, similar to the method described above.
When implementing riser installation and removal using these known procedures, it is necessary for the installation vessel to move close in to the platform (typically 20-40 m away) during the transfer of the topside end of the riser to the platform (FIG. 1), in order to maintain the integrity of the guide tube loads, maintain the bending radius of the riser, and prevent any contact between the riser and the port in the installation vessel through which it is lowered. The fact that the installation vessel must move so close to the platform is a significant disadvantage due to the risk of collision, especially with the riser platform. Indeed, many operators define an exclusion zone in the area adjacent to the riser platform, e.g. typically extending up 250 m away from the riser platform. If vessels do need to enter the exclusion zone, production and other operations must be ceased. Any cessation of production represents a significant expense.
It is noted that procedures similar to those used to install risers may be used to install other types of flexible lines, such as power cables and umbilicals. As such, the same problem is encountered, i.e. the need for an installation vessel to enter the exclusion zone and consequential shut down of operations.